In the composer, when the text cursor was inside a word starting
with a @ but not matching any username (thus the completion UI being
invisible), pressing up/down arrow would not do anything instead of
going to the prev/next line.
Send a pseudo-reply consisting of two messages: a `m.text` which is just
a reply with an empty body, then the file event itself.
This is a workaround to the restriction imposed by the Matrix API,
which prevents us from simply attaching a reply to a media event:
https://matrix.org/docs/spec/client_server/latest#rich-replies
These cause problems with the composer alias parsing code.
If an alias from the config file has whitespace, only the first word
will be taken into account (ignoring any leading or trailing whitespace).
Simplify the code, make it more declarative, and improve the reliability
of sending typing notifications (notably when changing the used alias in
the middle of a message).
If the chat was opened under e.g. "@user1:example.org", but the
composer text is "o abc" where "o" is a composer alias for
"@other_account:example.org", save the composer text to state.json as
"@user1:example.org" and not the alias user.
thumbnail() and download() were remaining.
Use a direct and carefully chosen MatrixClient's methods instead to
avoid problems mentioned in the previous commit 7502c1.
